In large enough amounts, yes. The United States and a few other countries use a massive dose of potassium chloride to carry out executions by lethal injection. Elemental potassium can also be deadly. It will react explosively with water, setting itself on fire and producing caustic potassium hydroxide.
The lethal dose for a human being varies upon the persons size, weight and tolerance. These are key factors in determining what would be a lethal dose as the larger a person is the more tolerant to the alcohol they will be. Anywhere from a litre upwards of vodka will be a lethal dose dependent on their size
Determining the lethal dose of any medication is pretty much impossible because of the differences between individuals and other chemicals such as alcohol that may be in the individual's system. The lethal dose is often quoted as the "minimum lethal dose" (MLD) or the least amount that will kill the most susceptible people. But, this value is determined with animal testing and extrapolated to humans. The MLD of Elavil is listed as 3 grams (120…
It's used as the third and final injection in the "lethal injection" method of execution in order to stop the heart. An certain quantity of potassium and chloride is essential for various biochemical functions in the body (particularly in the brain and muscles), but in large quantities it can be harmful. The oral LD50 is around 2500 mg/kg body weight. The intravenous lethal dose is much lower.